The present invention relates to a light distribution control device for varying the light distribution pattern of a headlamp, and more particularly to a light distribution control device for use with a headlamp which varies a clear-cut line in the light distribution pattern of the headlamp in accordance with the distance between the motor vehicle on which the headlamp is mounted and another motor vehicle traveling in front of the former.
The light distribution pattern of a conventional headlamp for a motor vehicle can be switched between a high beam pattern providing good visual recognition for long range illumination and a low beam having good visual recognition for short range illumination. The high beam is used when there is no vehicle closely in front or no approaching vehicle. When overtaking or meeting other vehicles, the light distribution pattern is switched to the low beam pattern, which has a clear-cut line so as to cause no glare to the driver of an approaching or close vehicle.
FIGS. 8 and 9 are graphical representations of variations of the amount of glare and the visual recognition distance in the case where two motor vehicles equipped with conventional sealed-beam headlamps of the double-bulb type pass each other on a straight road. FIG. 8 shows the amount of variation of glare perceived by a driver when receiving a light beam from the headlamps of another vehicle. (The amount of glare perceived is quantitatively expressed in terms of a quantity of glare VG with respect to distance relative to an approaching vehicle.) FIG. 9 shows the variation of visual recognition distance with respect to distance relative to an approaching vehicle. As seen from those figures, when two vehicles pass each other while using high beams, the glare quantity exceeds a tolerable quantity of glare VGa at approximately 300 meters of vehicle-to-vehicle distance (FIG. 8). Accordingly, an approaching driver perceives excessive glare. To avoid the glare, the driver in the other vehicle should switch the light distribution pattern from the high beam to the low beam.
If the driver fails to switch to the low beam, the approaching driver can be blinded by the glare. If vehicles pass each other under such conditions, a very dangerous situation is created. On the other hand, when the beam pattern is switched from the high beam to the low beam, the visual recognition distance is abruptly decreased from 100 meters to 70 meters, as seen from a graph of FIG. 9. This is also dangerous.